the Element % DMG is a part of the Weapon DMG equation while the Haunting Bonus is Weapon DMG x 2.
lets say your BWD is 2500 with Power 100 (7500 DMG), Item Quality +25% with Legend +200% WD, and +100% ED and Haunting Bonus.
(((2500 + 7500) x (1.25 x 3) x 2) x 2 =
((10,000 x 3.75) x 2) x 2 =
(37,500 x 2) x 2 =
75,000 x 2 = 150,000
without +100% Ed
((2500 + 7500) x (1.25 x 3) x 2 = 75,000
if you have a MH with Legend +200% WD, Haunting Bonus, and 4 Items with +100% Element DMG:
(10,000 x (1.25 x 3) x 5) x 2 = 375,000
Meteor is 750% Weapon DMG (using level 100 Gauntlet BWD)
(15,820 x (1.25 x 3) x 5) x 7.5 x 2 = 4,449,375.
maybe I should have said, any build should have at least one Legend +100% Element DMG for x2 DMG in the Weapon DMG equation (where the x5 is located). without Element % DMG, that would be x1.
if you are wondering about +5000 WD & +5000 ED, that is the Flat DMG part of the Weapon DMG equation in the Codex. these are added, not multiplied. the WD+ is added after WD% but multiplied by ED%. ED+ is added after ED%.
umm, the short answer is one Element % DMG is more important than Haunting Bonus, so using both is even better, especially since the Bonus gives both a DMG boost and +50% chance to increases DPS.
sorry about the long answer.